pop3 timeouts

I have ISPConfig with OpenSuse 10.2 installed by perfect setup guide. Everything worked fine for about 6 months. Now I have users which started to have timeouts when they uses pop3 e-mail clients and download messages with bigger attachments. They have to try several times before they can retrieve e-mail. When they connect to webmail they can retrieve e-mail without problem.
What could be the reason? There are a lot of hard disk space, CPU utilization is about 30%, there is enough RAM. I'have changed maxperip value in pop3d config file but nothing helped.
How can I see detailed pop3 log? Is it possible to trace connection to pop3 server? In mail log there is only who and when connected.